A variety of medical devices/medical articles are designed particularly for contact with a patient's bodily fluids. The duration of this contact may be relatively short, as is typical with devices used in endovascular procedures, or may be long term, which is typical with artificial organs implanted into the body of a recipient and with other implanted prostheses. Some devices, such as catheters, other percutaneous devices or components thereof, can have either short term or relatively long-term contact.
Prostheses, i.e., prosthetic articles, are used to repair or replace damaged or diseased organs, tissues and other structures in humans and animals. Prostheses generally are bio-compatible since they are typically implanted for extended periods of time. Examples of prostheses include, without limitation, prosthetic hearts, prosthetic heart valves, ligament repair materials, vessel repair and replacement materials, stents, and surgical patches. Development of artificial organs has been spurred by the sever shortage of donor organs. To date, artificial heart (implants), artificial liver (extracorporeal) and artificial kidney (extracorporeal) have been used in the clinic.
Contact of articles with bodily fluids creates a number of functional and biocompatibility challenges. Some challenges associated with these medical devices are as follows; blood compatibility, infection control, rejection, and nutrient transfer (i.e. passing oxygenated blood to devices, or transport of fluid on blood for bio-active benefit). All these challenges can reduce the relative degree of success of the medical articles and corresponding medical procedures, for example, to repair, treat or replace an injured or diseased native structure.
When blood compatibility is not achieved, an interaction between a medical device and the patient's bodily fluids can result in a complex cascade of events creating a biological reaction, such as aggregated thrombus formation or “clot.” Thrombus formation can be the first event a medical device experiences when placed into contact with the blood. This biological reaction can be in response to a toxic substance or to an irregular surface structure, which causes red blood cells to lyse thereby inducing a nexus for blood clotting and non-specific fibrin aggregation on the material. This primary event can precipitate secondary diseases states such as embolization, fibrolitic overgrowth, restenosis, and calcification. The secondary diseases can be a relatively early event occurring within minutes to hours after implantation. While very important, blood compatibility is not the only biological complication that can result from use of a medical article. For example, devices that are very blood compatible may still be faced with problems relating to infection.
Introduction of medical articles or devices into contact with bodily fluids creates an increased a risk of bacterial colonization. These infections can affect the function of the medical article and may cause increased morbidity and mortality, which may or may not be related to the function of the medical device. For example, infection can affect the adhesive properties of a glue, which can result in failure of a medical device. Beyond mechanical and functional considerations, often contaminated devices and the surrounding tissue are removed to prevent pailure of the device and/or spread of the infection. In the worst cases, sepses sets in and the entire system is compromised. If the patient contracts sepses, the patient can die of secondary organ failure. These concerns are further propagated by the existence of antibiotic resistant strains of bacteria.
The body uses the biological mechanisms brought to bear by the immune system to deal with infections. Unfortunately, the immune system cannot necessarily distinguish harmful foreign invaders from helpful medical articles. The attack of medical devices or prosthetic organs by the immune system is termed “rejection”.
In the immune system, each protector cell (e.g., B cells, Macrophages, and T cells) in the body is genetically programmed to recognize a specific molecular marker on an invading material or organism (antigens). When one of these cells encounters a mismatched antigen, the cell attempts to engulf the foreign material and expresses the antigen motifs on its cell surface. When the response is complete, suppressor T cells help shut down the immune response, but when the material is not resorbable or too large, a chronic immune reaction or rejection of the material can ensue. Rejection can necessitate treatment with expensive drugs that also may have undesirable side effects. In extreme cases, rejection can necessitate removal of the medical device.
Another challenge of device implantation can be lack of nutrient transport. This is a particular challenge for tissue engineering and organogenesis where complex three dimensional structures are fabricated that may require not only structural integrity and functioning architecture but also require delivery of biologically essential fluids, e.g., oxygenated blood to the cells integral to the structure. Efforts to promote vascularization or other types of fluid transport systems for medical devices has seen limited success.
These challenges are at the corner stone of device and materials interaction, and the appropriate handling of these challenges can be helpful in making better, longer lasting, more successful devices to provide treatment or to replace diseased or injured body parts.